Method for preparing rare earth oxide phosphors

ABSTRACT

Rare earth phosphors with excellent optical and screening properties and optimum particle size distribution for the preparation of color television tubes are prepared by treating rare earth salts such as oxalate, tartrate or sulfate containing at least one rare earth metal, and an activator, with a caustic solution, firing the caustic treated rare earth salts to high temperatures in a non-reducing atmosphere to form oxides, and thereafter wet milling, washing, and drying to prepare said phosphors. The phosphors thus obtained contain a small amount of retained alkali up to about 3 percent by weight, at least a major portion of which can be removed by the sequential treatment including the steps of refining, washing, neutralizing the slurry with acid and drying. The preferred phosphors have the general formula

United States Patent 91 Byler et al.

[451 Feb. 20, 1973 [54] METHOD FOR PREPARING RARE EARTH OXIDE PHOSPHORS[75] Inventors: William H. Byler, Landing lames J.

Maids,lsong'valle'y'fboth oTNJ.

[73] Assignee: U.S. Radium Corporation [22] Filed: June 11, 1970 [21]Appl. No.: 45,402

Related U.S. Application Data [63] Continuation-impart of Ser. No.685,219, Nov. 22,

1967, abandoned.

3,273,806 9/1966 Aokietal. 252 301.4x

Primary Examiner--Oscar S. Vertiz Assistant Examiner-J. CooperAttorneyPennie, Edmonds, Morton, Taylor and Adams 57 ABSTRACT Rare earthphosphors with excellent optical and screening properties and optimumparticle size distribution for the preparation of color television tubesare prepared by treating rare earth salts such as oxalate, tartrate orsulfate containing at least one rare earth metal, and an activator, witha caustic solution, firing the caustic treated rare earth salts to hightemperatures in a non-reducing atmosphere to form oxides, and thereafterwet milling, washing, and drying to prepare said phosphors. Thephosphors thus obtained contain a small amount of retained alkali up toabout 3 percent by weight, at least a major portion of which can beremoved by the sequential treatment including the steps of refining,washing, neutralizing the slurry with acid and drying. The preferredphosphors have the general formula wherein R is Gd, ,,Y,,, x is a valuefrom 0.02 to 0.10,

l m and n each is a value from zero to l and (Li ,,,Na,,,)

is the retained alkali, the weight of which is preferably less thanabout 1.5 percent.

12 Claims, Drawing Figures PATENTEHE Z SHEET 10! 2 FIG.|

lNvENroRs dbl/Mg, 14 0 n-u. f M;

FIG. 2

ATTORNEY METHOD FOR PREPARING RARE EARTH OXIDE PHOSPI-IORS RELATEDAPPLICATION This application is a continuation-in-part of our copendingapplication Ser. No. 685,219, filed Nov. 22, 1967 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to rare earth phosphors, and more particularly to a method forpreparing the phosphors (generally in association with a rare earthactivator) which involves a caustic treatment, and to the resultant rareearth phosphors (and associated activator) with retained alkali metal ormetals. The term rare earths as used in the present specification refersto yttrium and scandium plus the metals in Group III of the periodictable generally classified as lanthanide rare earths, to wit: lanthanum,cerium, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.The term phosphor refers to a material which is capable of exhibitingluminesence when subjected to appropriate excitation. The term rareearth activator refers to compounds of rare earth elements which may becombined with other rare earth compounds to activate luminesence thereofand used as activators, including, for example, compounds of europium,terbium, erbium, thulium, dysprosium, ytterbium, praseodymium andgadolinium.

2. Description of the Prior Art Luminescent properties of certain rareearth containing compositions have long been recognized. In recentyears, rare earth phosphors have received considerable attention and arethe subject of many intensive investigations. The recent interest is duepartly to the discovery that certain rare earth phosphors, particularlythe oxygen bearing phosphors (oxyphosphors) can be used advantageouslyas cathode-luminescent coatings for color television tubes. In general,the rare earth phosphors are in the form of a solid solution having amatrix of rare earth compounds such as a rare earth oxide or vanadateand an activator which is commonly called a dopant and generally is alsoa rare earth element. The phosphor usually contains a trace amount ofother elements which may be impurities from the starting materials oraccidentally introduced into the phosphor during its preparation orwhich may be added thereto purposefully to improve the properties of thefinal phosphor.

The effectiveness of the activator is dependent to a large extent on itsintimate relation within the rare earth matrix. To ensure theformulation of an intimate mixture, rare earth phosphor manufacturersmay prefer to dissolve into an acid solution the rare earth element inthe form of an oxide, together with the activator, to form a homogeneoussolution. The rare earth element and the activator are thencoprecipitated out in the form of oxalates, hydroxides or carbonates.The precipitates are recovered and fired to a high temperature todecompose the salts into mixed oxides in powder form. This finelydivided, reactive form is favorable for reaction with certain acidoxides to synthesize such oxygen-dominated and europium-activatedphosphors as yttrium vanadate, gadolinium vanadate, yttrium tungstate,yttrium germanate, gadolinium aluminate, etc., and conditions canbeadjusted to yield desirable crystal growth and particle sizedistribution. The phosphors thus synthesized may be used as luminescentcoatings for color television tubes and other applications.

Certain oxide phosphors prepared by the aforesaid precipitation anddecomposition method are also efficient luminescent materials, whenproperly excited, with emission of visible light in the desirable rangeof wave length. Due to the small crystal size of the oxide phosphors,they are difficult to handle and are limited in their applications.Attempts to grow the oxide phosphors by increasing the firingtemperature and duration have not been successful. Such treatment doesnot suffice to control the crystal growth properly, and leads to wideparticle size distribution, or results in crystal growth at the expenseof lowering the phosphors optical properties. Failure to obtain controlled crystal growth also limits application of the oxide phosphors asprephosphors for preparing some other types of rare earth phosphors(such as the preparation of rare earth oxysulfide phosphors) whensubsequent steps for such preparation does not favor desirable crystalgrowth.

SUMMARY OF INVENTION We have discovered that controlled crystal growthfor rare earth oxide phosphors can be obtained with the process of thisinvention. Broadly stated, the process comprises the steps of treating arare earth salt intimately mixed with an activator with a causticsolution, preferably lithium, sodium or potassium hydroxide solution,and thereafter subjecting the thus treated salt to a high temperaturetreatment in a preselected atmosphere to form the phosphor. The rareearth salts suitable for the process of this invention are those capableof being decomposed to form the corresponding rare earth oxide.

Preferably, these rare earth oxide phosphors are prepared by the processwhich comprises the steps of preparing a homogeneous solution of atleast one rare earth metal and an activator. The rare earth metal andthe activator preferably are then coprecipitated. After the precipitatesare recovered, they are treated with a caustic solution. The thustreated precipitates are then heated in a preselected atmosphere to formthe phosphor.

The firing is carried out in an essentially non-reducing atmosphere andin sequential steps including firing the caustic treated precipitatesinitially at a temperature sufficiently high for the formation ofoxides, breaking up any aggregates formed in the initial firingoperation, washing and drying the oxide, and thereafter retiring the dryoxide to above about 1000 F. The caustic solution for treating theprecipitates preferably is a solution of NaOI-l, LiOH, KOH or a mixtureof these alkali hydroxides.

The rare earth phosphors of the invention contain a small amount ofretained alkali metal from the caustic solution used in the caustictreatment, preferably lithium, sodium, or potassium, or a mixturethereof. They exhibit controlled grain growth properties and optimaloptical properties. They comprise a rare earth oxide matrix having arare earth activator therein, and a small amount of retained alkalimetal which is in an amount ranging from 0.002 percent in the case oflithium or from 0.006 percent in the case of sodium or potassium up to1.5 percent by weight of the phosphor. These phosphors have an averagecrystal size in the range from 3 to 30 microns.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying two sheets ofdrawings show photomicrographs of europium-activated gadolinium oxidecrystals.

The crystals shown in FIG. I have retained alkali in the matrix and wereprepared according to a procedure to be described hereinafter in ExampleI.

The crystals shown in FIG. 2 do not have retained alkali in the matrixand were prepared in a similar manner as the crystals in FIG. 1 with theexception that the caustic treatment was omitted.

The crystals shown in FIG. 3 also have retained alkali in the matrix andwere prepared in accordance with a procedure described in Example II.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Any of the rare earth elements(as defined above) may be used in preparing oxide phosphors inaccordance with the invention, provided they are used in a form capableof functioning as a phosphor. That is to say, they must yield an oxidewhich is transparent or translucent to the wave length of the radiationemitted. For example, very dark or black oxides such as the oxide oftetravalent praseodymium are not satisfactory for phosphors which are toemit visible light. The selection of suitable rare earth elements forphosphors for particular uses is well within the competence of phosphorchemists; but in general the use of the light colored oxides of thetrivalent rare earth elements lanthanum, gadolinium, lutetium, yttriumand scandium are preferred.

Similarly, many of the rare earth elements (as defined above) may beused as activator in a rare earth phosphor according to the invention.Not all rare earth elements may be used to activate any rare earth oxidephosphor to emit radiation of any wave length, but those that may beused to activate one of the phosphors specified above includepraseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, ytterbium, and thulium. The selection ofparticular activators for particular phosphor compositions to promoteemission of radiation of particular wave lengths is not a part of thisinvention. Rather, the invention draws on the knowledge of the art tomake such selection. So far as this invention is concerned, any suitablerare earth activator or any suitable combination of them may be used.

To carry out the process of this invention the initial steps of formingan intimate mixture containing the rare earth metal values for thephosphor and the activator cations in predetermined proportions may beaccomplished by conventional techniques well known to one skilled in theart. Thus, one may obtain a homogeneous solution by dissolving thereinthe soluble compounds of rare earth elements as well as the activator.In general, the oxides, sulfates, nitrates and chlorides are the rareearth compounds most commonly used to prepare the homogeneous solution.Because of the relative insolubility of the rare earth oxides, acidicsolutions are generally used for preparing the homogeneous solutions. Weprefer to use inorganic mineral acids such as hydrochloric acid,sulfuric acid and nitric acid for this purpose.

The amount of rare earth salt and activator that is dissolved in thesolvent is not critical. The maximum amount of rare earth that can beused is generally governed by the solubility of the compound in thesolvent. When a mineral acid is used as a solvent, the concentration ofthe acid dictates the concentration of the solutes therein.

Usually, the higher the concentration, the smaller the volume requiredto dissolve a certain amount of rare earth compounds and activators. Wefound it to be advantageous to obtain an acidic solution with a pH inthe range between 1 and 4 if the precipitate subsequently obtained is tobe in the form of an oxalate.

Advantageously the proportion of the rare earth and the activator in thesolution is in the same stoichiometric ratio as in the phosphor. Theamount of activator in the phosphor varies within a wide range dependingon a number of factors. For some of the rare earth oxide phosphors, theamount by weight may be within the range of0.02 to 0.10.

The rare earth element and the activator are coprecipitated from thehomogeneous solution by adding thereto a precipitating agent well knownin the art. Thus, the coprecipitation may be accomplished by adding tothe solution oxalic acid, tartaric acid, and ammonium carbonate toprecipitate therefrom oxalates, tartrate and carbonate, respectively.

In the process of this invention, we prefer to use oxalic acid tocoprecipitate the desirable metal values fro an acidic solution. Thismay be performed, for example, by adding an oxalic solution to theacidic solution containing the rare earth values and the activator. Themixing of oxalic with the acidic solution may be carried out over a widetemperature range, e.g., 10C. C. The pH of the resultant aqueousmixture, however, should be adjusted so that it is below the value atwhich the rare earth hydroxide precipitates are formed. In mostinstances the pH is advantageously kept at a lower value which canpreferentially precipitate the rare earth oxalate and not theimpurities; thus, it is an effective means for controlling theimpurities in the final composition of the phosphor.

The initial concentration of the acidic solution, the amount of theoxalic acid added thereto, the pH of the final solution and thetemperature for the coprecipitation all have influences on the type ofcrystals obtained and the size of the crystals. In general, it isadvantageous to adjust these variables so as to allow a coprecipitationthat will produce phosphors with optimal properties.

Instead of oxalic acid, tartaric acid may be used effectively forcoprecipitation of rare earth values and the activator. When tartaricacid is used, however, the pH should be slightly higher than that usedwith the oxalic acid. The pH value required, nevertheless, is stillbelow the value at which the rare earth metal values are precipitated ashydroxides.

The precipitates thus obtained are recovered by filtration and followedby washing. These precipitates which are a mixture of salts containingrare earth cations and ions of the activators, are now ready for caustictreatment.

Although the invention has been described particularly with reference tocoprecipitation of the rare earth phosphor and activator, otherprocedures for making an intimate mixture of these substances may beemployed. For example, the phosphor rare earth and of the activator rareearth may be cocrystallized from the solution, yielding a crop ofcrystals in which the phosphor and activator are intimately mixed insuitable proportions. The cocrystallized product may then be subjectedto the caustic treatment which characterizes the method of theinvention.

Advantageously, the treatment is performed by reslurrying theprecipitates or crystals in a caustic solution. In the case of crystals,the volume of caustic solution should be limited to minimizeredissolving of the crystals. While any caustic solution has beneficialresults in promoting crystal growth in subsequent firing operations, thespecific alkali hydroxide or mixture of alkali hydroxides is significantin determining the final crystal size and the optical properties of thephosphors subsequently recovered. Lithium hydroxide, sodium hydroxide,and potassium hydroxide are the caustics which in general are used, butthe invention does not exclude the use of the relatively rarer alkalicaustics (e.g., rubidium hydroxide and cesium hydroxide). In general,lithium hydroxide is most effective in promoting crystal growth, andsodium hydroxide and potassium hydroxide are relatively less so.

The selection of the caustic solution, concentration, and temperaturefor the treatment is dependent also on the rare earth used. For example,it is easier to grow lanthaum oxide crystals than gadolinium oxidecrystals and the latter is in turn easier to grow than yttrium oxidecrystals. It is, therefore, apparent that for a given crystal growth,the caustic treatment for lanthanum is less severe in comparison withthe requirement for yttrium. The selection of variables for the caustictreatment, however, is not completely based on the crystal growth. Thecaustic solution and the treatment conditions should be chosen toprovide, in addition to crystal growth, optimal optical properties.

The selection of variables in the caustic treatment may be guided to acertain extent on chemical grounds. For example, lanthanum is more basicthan gadolinium which leads to differences in solubility, in size ofprecipitated crystals under given conditions, in reactivity of oxalatewith caustic, in decomposition temperature, in crystal habit and ingrowth rate. Because gadolinium and yttrium oxides are less basic, theytend not to grow as readily as lanthanum oxide; hence conditions for thecaustic treatment are different from those for lanthanum and they areselected to favor growth and optimum retention of alkali. The alkali oralkali metals selected must also not hinder but be favorable in relationto phosphor brightness. Thus, we found lithium hydroxide which providesthe best crystal growth is favored for yttrium oxide and sodiumhydroxide which is not as effective as LiOH for promoting crystalgrowth, may be used alone or in combination with lithium hydroxide orpotassium hydroxide for the growth of gadolinium oxide crystals.

As briefly described hereinabove, it is possible to use a causticsolution containing a mixture of alkali hydroxides. For example, wefound a mixture of lithium hydroxide and sodium hydroxide is effectivefor controlling the crystal growth in a mixed oxide phosphor such as ina Gd O -Y O Eu O system without having a deleterious effect on theluminescent efficiency of the resultant phosphors. Indeed, it ispossible to prepare phosphors across the whole spectrum of the Gd O Y Omixture and to control the particle size by the adjustment of theLiOHNaOI-I ratios in the caustic solution which leads to the formationof the phosphor of the following formula in which m and n each can varyfrom zero to 1, [Li Na is the retained alkali in an amount less thanabout 1.5 percent by weight and x is in the range between 0.02 to 0.1.

In addition to the specific influence of a particular alkali hydroxideor a mixture of alkali hydroxides on the crystal growth and opticalproperties of the oxyphosphors subsequently recovered, the concentrationof the caustic solution, and the duration and temperature of thetreatment also have definite effects on the final properties of thephosphors. The rule to follow is that high concentration and temperatureand a longer treatment period promote greater crystal growth. Forcommercial production we found a caustic solution containing 5 to 35percent by weight of alkali hydroxide or hydroxides in relation tocontained rare earth oxides may be advantageously added to the slurry inthe caustic treatment step of the process carried out at a temperaturein the range between 20C. and C. and for a duration between 20 minutesand 5 hours. The final caustic concentration of the slurry may be in therange between 1 to 10 percent by weight. Subsequent to the caustictreatment, the precipitate is again recovered by filtration. The washedcrystals are then dried and are now ready for firing to form the oxidephosphor.

To promote optimal crystal growth, we prefer to first decompose thecrystal to form the oxide and then fire a second time at a highertemperature. The temperatures are raised to l0OO to l500F. for a periodof l to 3 hours which causes the precipitates to decompose andthereafter the temperature is further raised to 2000 to 2400F. for 2 to10 hours to insure complete conversion of the oxalates to oxides and togrow the crystal to the desired size range.

After the high temperature firing, the fine oxide crystals are in theform of relatively large loosely bonded agglomerates. Wet milling gentlywith water breaks up the agglomerates. After milling the oxide crystalsin powder form are repeatedly washed.

For the conversion of rare earth tartrates to oxides, similar firingprocedures may be followed. The firing and refiring temperatures anddurations again depend on the rare earth oxides and the ultimate crystalsize. It should be noted that for both oxalates and tartrates, thefiring temperatures are not very critical. The temperatures, however,should be sufficiently high for the purposes of conversion and crystalgrowth.

If the precipitates are rare earth carbonates, which decompose to formrare earth oxides in a similar manner as oxalates as depicted belowIntermediate R the above-described firing procedure can beadvantageously used after caustic treatment to convert the carbonates tooxides and to promote crystal growth. The selection of the firingtemperatures should be compatable with the rare earth element and to thefinal crystal size desired.

After the final refiring operation, the phosphor is preferably washedwith water and the slurry is neutral ized with a dilute acid. Thewashing and neutralizing steps are used to insure that all alkali whichis not retained in the phosphor is removed or otherwise neutralized.

The rare earth oxide phosphors in accordance with this invention have,as previously stated, retained alkali in their matrix. The precisemanner in which the alkali is associated with and bonded to the rareearth oxide matrix is not completely clear. However, we have found thatthe alkali which is retained with the rare earth oxide is important toachieve controlled crystal growth and optimum phosphor brightness. Theexact amount of retained alkali which is necessary to impart thesuperior properties to the rare earth phosphor varies. Generally, wehave found that the amount of retained alkali may be as low as 0.002percent in the case of lithium, or 0.006 percent in the case of sodiumor potassium, by weight of the phosphor, or as high as 1.5 percent byweight of the phosphor. However, an amount of retained alkali in therange of about 0.005 to 0.012 percent in the case of lithium, or 0.06 to0.13 percent in the case of sodium, or 0.02 to 0.09 percent in the caseof potassium, by weight of the phosphor in each case is eminentlysuitable and is in general preferred.

To illustrate the superior properties of the rare earth oxide phosphorswith retained alkali as compared with rare earth oxide phosphors withoutretained alkali, samples of both such phosphors were coated on aconventional cathode tube and were tested for brightness. The rare earthoxide phosphors with retained alkali showed an average brightness 11 to16 percent greater than the rare earth oxide phosphors without theretained alkali under the same excitation conditions.

The exact amount of alkali retained in the phosphor is determined bymany factors including the concentration of the caustic solution used toprepare the phosphor, the duration and temperature of the caustictreatment, the subsequent firing and retiring of the temperatures, thenumber of retiring cycles and the type of phosphor which is used. Ingeneral, we found the amount of retained alkali may be as high as 1.5percent by weight of the phosphor. This large amount of alkali, however,is still far below that which is required to form an alkali metal saltof the formula ARO wherein A is an alkali metal such as lithium orsodium and R (as above) is a rare earth or a mixture of rare earths suchas a mixture of europium and gadolinium or a mixture of yttrium andeuropium. Such alkali metal compounds (e.g. lithium yttrate and sodiumgadolinate) have been proposed for use as rare earth activatedphosphors, but have been found to be unstable (J. Electrochem. Soc.,114, 252-, same 1., 116, 663).

We have run many experiments using the process of this invention toprepare oxide phosphors. These experiments show that alkali is retainedin these phosphors after the initial high temperature firing, e.g.,2150F. and that a substantial fraction of this alkali is released byretiring one or more times at a lower temperature, e.g., 1400 l800F.Following wet milling, washing and drying. The amount of released alkaliis determined by re-slurrying the phosphors in water and titrating themwith dilute nitric acid. Samples of similar phosphors which have notbeen subjected to caustic treatment show no excess alkalinity whereasthe caustic treated samples have shown amounts up to as high as 3percent by weight of total alkali released by repeat firing. Thephosphor as it stands after the high temperature firing, wet milling,washing, and drying may be an efficient one but it is found generallythat improvement results from retiring. Repeated retiring in some casesreleases additional alkali and may still further improve efticiency butthis is not necessarily true. In all cases, atomic absorption analysisshows significantly more alkali retained than control samples preparedwithout the use of caustic. The following are some atomic absorptionanalysis results:

TABLE I 1. Gd,O Eu (non-caustic control) l 2. Gd,0,: Eu (non-caustic butwith (PP (PP" 48 i0 Samples 3, 4, S, 6 and 8 were retired to the pointof essentially no further release of alkali as shown by titration.

Sample 2 was processed the same as sample 1 except 10% Na* was added asNaNO to the rare earth solution before precipitation. This sample hadthe same inferior crystal development and optical properties assample 1. In samples 5 and 9 the LiOl-l used for the caustic treatmentcontains a small amount of NaOl-l as impurity (less than 1 percent byweight) which account for the sodium retention.

Samples 3 and 4 illustrate the sodium retention of two typical caustictreated phosphors, each being substantially higher than the sodiumretentions of the noncaustic treated samples 1 and 2. In sample 1, thesodium therein was due to sodium impurities in the starting materials orto the contamination in subsequent process whereas in sample 2 thesodium was purposefully introduced thereto.

While the exact bonding of the retained alkali in the phosphor isuncertain, the beneficial results in terms of higher optical efficiency(brighter) with controlled crystal growth are quite apparent asillustrated in the following examples:

EXAMPLEI Gd O (99.9 percent quality) was dissolved in hydrochloric acidto prepare a 0.6 molar solution and the pH of the final solution wasadjusted to about 2 (using ammonium hydroxide when needed). Eu O (99.9percent quality) was dissolved in hydrochloric acid to prepare a 1.0molar solution and the pH of the final solution was similarly adjustedto about 2. Gd and Eu solutions equivalent to 450 grams of mixed oxidesat about 6 mol percent Eu O were mixed and the temperature was adjustedto 30C. While stirring, a 20 percent excess of a l percent oxalic acidwas added to the Gd-Eu mixed solution at a steady rate over an 8-minuteperiod and stirring was continued for 10 minutes.

The coprecipitates were filtered and washed with water. The washedcoprecipitates were reslurried in 2250 milliliters of water in whichwere added 450 milliliters of 25 percent by weight NaOH solution. Theresultant slurry was stirred intermittently for a period of 1 hour. Thecaustic treated crystals were recovered by filtration, followed bywashing with water. The washed crystals were dried for 16 hours at 300F.and were fired for 1 hour at 1300F. The temperature was raised to 2150F.and was maintained at that temperature for 3 hours.

The fired crystals in the form of loosely held agglomerates were wetmilled gently with water for 40 minutes. Thereafter the wet crystalswere wet sieved to remove uncomminuted agglomerates and were washed anddried. The wet milled and dried crystals were refired for 2 hours at1600F.

The photomicrogram of the europium activated gadolinium oxide of ExampleI is shown in FIG. 1. The small division of the scale appearing thereonis equal to 3.1 microns.

In a control sample, the phosphors were prepared in the identical manneras described hereinabove with the exception that caustic treatment wasomitted. The resultant crystals are shown in FIG. 2 of the accompanyingdrawing.

Caustic treated and untreated phosphors were coated on a conventionalcathode tube and were tested for brightness. It was found that thecaustic treated phosphor was 11 to 16 percent brighter than theuntreated material under comparable operating conditions. An untreatedsample which showed only 89 percent of the brightness of the causticcontrol was on the average only about 20 percent as large in crystalsize. When a portion of this untreated sample was refired at a highertemperature and for a longer period, the crystal size grew to 61 percentof that of the treated sample, but the brightness dropped to 42 percent.It is apparent that without caustic treatment preparation of phosphorswith properly combined properties was not feasible.

EXAMPLE II The oxalate precipitate prepared in a manner similar toExample I was treated with a LiOl-I solution containing a small amountof NaOH impurity therein. After treatment, the phosphor was prepared inthe same manner as in Example I. The phosphor obtained has a greatercrystal growth as shown in FIG. 3 of the accompanying drawings.

EXAMPLE III Y O (99.9 percent quality) was dissolved in hydrochloricacid and the resultant solution was adjusted to pH 2, to make up a 0.6molar solution. Yttrium and europium solutions equivalent to 450 gramsof mixed oxide at 5 mol percent Hu o: were mixed and the temperature wasadjusted to 30C. While stirring a 20 percent excess of a 10 percentoxalic acid was added thereto at a steady rate over a 13-minute periodand the stirring was continued for an additional 10 minutes. Theprecipitates were filtered and washed with water. The recoveredprecipitates were reslurried in 2250 ml. water and into which were addedunder constant stirring 450 ml. 20% LiOl-l plus 99 ml. 25% NaOH. Over aperiod of 1 hour the slurry was stirred occasionally. After filtration,the phosphor was dried for 16 hours at 260F. After drying it was firedin air for 1 hour at 1300F. and an additional 3 hours at 2l50F. Thefired phosphor was wet milled, sieved and dried and thereafter it wasrefired at l600F. for a 2-hour period.

The caustic treatment process is not limited to those rare earth saltsprepared by precipitation. An oxide prepared by decomposing gadoliniumsulfate crystals which had been caustic treated also showed desirablecrystal growth and size distribution while an untreated portion of thesalt showed poor crystallinity.

In addition to the above specifically described phosphors, other oxidephosphors may be prepared using the process of this invention. For theeuropium activated phosphors in general, they can be represented by thegeneral formula wherein R is at least one rare earth element selectedfrom the group consisting of scandium, yttrium, lanthanum, gadoliniumand lutetium, x is 0.02 to 0.1 and M is retained alkali metal or metalsin the rare earth matrix, generally below about 0.5 percent by weight ofthe phosphor.

It is known that efficiency of rare earth phosphors can be affected byaddition of certain rare earth impurities, usually in trace amounts, theeffect being favorable in some cases and unfavorable in others. Thesetrace effects may be significant even in amounts commonly found incommercial high grade rare earth compounds. Samarium has long been knownto produce emission on the red side of the spectrum in a wide variety ofphosphors. Certain non-rare earth elements (such as bismuth whichproduces red emission in a number of phosphors) are known also to behyperactive. Trace amounts are usually incorporated without gross effecton structure such as could be revealed by X-ray diffraction analysis.Our basic process for the rare earth oxide phosphors has advantages overprior art whether or not trace impurities are present.

We claim:

1. A method for preparing a rare earth oxide phosphor which comprises:

treating an intimate mixture of a salt of a rare earth metal selectedfrom the group consisting of lanthanum, gadolinium, lutetium, yttriumand scandium and a rare earth activator selected from the groupconsisting of praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, ytterbium, and thulium, said saltbeing capable of being decomposed to form corresponding rare earth oxidea. with an alkali metal caustic solution containing from about 5 percentto 35 percent alkali by weight of said rare earth oxide b. at atemperature of about 10C to C for about 20 minutes to 5 hours,

firing the caustic treated salt in an essentially nonreducing atmosphereto decompose said salt to form said oxide phosphor; and

thereafter retiring the dry oxide to above about 2. A method accordingto claim 1 wherein prior to refiring, the fired rare earth oxidephosphor is comminuted, washed and dried.

3, A method according to claim 1 wherein the caustic solution is atleast one of a group consisting of LiOI-i, NaOH and KOH solutions.

4. A method for preparing a rare earth oxide phosphor which comprises:

preparing a homogeneous solution of a salt of a rare earth metalselected from the group consisting of lanthanum, gadolinium, lutetium,yttrium and scandium and a salt of a rare earth activator selected fromthe group consisting of praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, ytterbium, andthulium; coprecipitating a salt of said rare earth metal with saidactivator, said coprecipitate being capable of being decomposed to formcorresponding rare earth oxides; recovering the precipitates; treatingthe precipitates thus recovered at a temperature of about C to 70C forabout minutes to 5 hours with an alkali metal caustic solutioncontaining from about 5 percent to 35 percent alkali by weight of saidrare earth oxides;

firing the caustic treated precipitates in an essentially non-reducingatmosphere to form said oxide phosphor; and

thereafter retiring the dry oxide to above about 5. A method accordingto claim 4 wherein prior to retiring, the fired rare earth oxidephosphor is comminuted, washed and dried.

6. A method according to claim 4 wherein the caustic solution is atleast one of a group consisting of LiOH, NaOH and KOH solutions,

7. A method for preparing a rare earth oxide phosphor which comprises:

preparing an acid solution of at least one salt of a rare earth metalselected from the group consisting of lanthanum, gadolinium, lutetium,yttrium and scandium and a salt of a rare earth activator selected fromthe group consisting of praseodymiurn, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, ytterbium, andthulium; coprecipitating said rare earth metal with said activator inthe form of oxalates or tartrates at a pH below about that required toprecipitate the rare earth metal in hydroxide form, said precipitatebeing capable of being decomposed to corresponding rare earth oxides;

recovering the precipitate;

treating the precipitate, at a temperature from about 10C to C for about20 minutes to 5 hours with an alkali metal caustic solution containingfrom about 5 percent to 35 percent alkali by weight of said rare earthoxides;

firing the thus treated precipitate in an essentially non-reducingatmosphere to form said oxide phosphor and thereafter retiring the driedoxide to above about 1000F. 8. A method according to claim 7 whereinprior to refiring, the fired rare earth oxide phosphor is wet milled,washed and dried.

9. A method according to claim 7 wherein the rare earth metal isyttrium, gadolinium or a mixture thereof.

10. A method according to claim 9 wherein the activator is europium.

11. A method according to claim 9 wherein yttrium, gadolinium or amixture thereof is coprecipitated with the activator as oxalates and theoxalate thus recovered is treated with a caustic solution containingNaOH, KOH or LiOH or a mixture thereof.

12. A method according to claim 11 wherein prior to retiring, the tiredrare earth oxide phosphor is wet milled, washed and dried.

1. A method for preparing a rare earth oxide phosphor which comprises:treating an intimate mixture of a salt of a rare earth metal selectedfrom the group consisting of lanthanum, gadolinium, lutetium, yttriumand scandium and a rare earth activator selected from the groupconsisting of praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, ytterbium, and thulium, said saltbeing capable of being decomposed to form corresponding rare earth oxidea. with an alkali metal caustic solution containing from about 5 percentto 35 percent alkali by weight of said rare earth oxide b. at atemperature of about 10*C to 70*C for about 20 minutes to 5 hours,firing the caustic treated salt in an essentially non-reducingatmosphere to decompose said salt to form said oxide phosphor; andthereafter refiring the dry oxide to above about 1000*F.
 2. A methodaccording to claim 1 wherein prior to refiring, the fired rare earthoxide phosphor is commiNuted, washed and dried.
 3. A method according toclaim 1 wherein the caustic solution is at least one of a groupconsisting of LiOH, NaOH and KOH solutions.
 4. A method for preparing arare earth oxide phosphor which comprises: preparing a homogeneoussolution of a salt of a rare earth metal selected from the groupconsisting of lanthanum, gadolinium, lutetium, yttrium and scandium anda salt of a rare earth activator selected from the group consisting ofpraseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, ytterbium, and thulium; coprecipitating asalt of said rare earth metal with said activator, said coprecipitatebeing capable of being decomposed to form corresponding rare earthoxides; recovering the precipitates; treating the precipitates thusrecovered at a temperature of about 10*C to 70*C for about 20 minutes to5 hours with an alkali metal caustic solution containing from about 5percent to 35 percent alkali by weight of said rare earth oxides; firingthe caustic treated precipitates in an essentially non-reducingatmosphere to form said oxide phosphor; and thereafter refiring the dryoxide to above about 1000*F.
 5. A method according to claim 4 whereinprior to refiring, the fired rare earth oxide phosphor is comminuted,washed and dried.
 6. A method according to claim 4 wherein the causticsolution is at least one of a group consisting of LiOH, NaOH and KOHsolutions.
 7. A method for preparing a rare earth oxide phosphor whichcomprises: preparing an acid solution of at least one salt of a rareearth metal selected from the group consisting of lanthanum, gadolinium,lutetium, yttrium and scandium and a salt of a rare earth activatorselected from the group consisting of praseodymium, neodymium, samarium,europium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium,and thulium; coprecipitating said rare earth metal with said activatorin the form of oxalates or tartrates at a pH below about that requiredto precipitate the rare earth metal in hydroxide form, said precipitatebeing capable of being decomposed to corresponding rare earth oxides;recovering the precipitate; treating the precipitate, at a temperaturefrom about 10*C to 70*C for about 20 minutes to 5 hours with an alkalimetal caustic solution containing from about 5 percent to 35 percentalkali by weight of said rare earth oxides; firing the thus treatedprecipitate in an essentially non-reducing atmosphere to form said oxidephosphor and thereafter refiring the dried oxide to above about 1000*F.8. A method according to claim 7 wherein prior to refiring, the firedrare earth oxide phosphor is wet milled, washed and dried.
 9. A methodaccording to claim 7 wherein the rare earth metal is yttrium, gadoliniumor a mixture thereof.
 10. A method according to claim 9 wherein theactivator is europium.
 11. A method according to claim 9 whereinyttrium, gadolinium or a mixture thereof is coprecipitated with theactivator as oxalates and the oxalate thus recovered is treated with acaustic solution containing NaOH, KOH or LiOH or a mixture thereof.